This invention relates to signal processing techniques and, more particularly, to the processing of an electrical signal derived from an acoustic signal generated in a test object.
The electromagnetic acoustic transducer (EMAT) is a recently developed device which has generated particular interest in the field of nondestructive evaluation because an EMAT may be operated in a noncontact mode, thereby enabling a high rate of ultrasonic inspection to be achieved with an attendant reduction in the costs of quality control. The noncontact feature of such a transducer is a consequence of the basic principles of operation of an EMAT, which depend upn the interactions between a magnetic field and an alternating current. When a conductor carrying a dynamic current is placed adjacent to a metallic object, eddy currents are induced within the object by the electromagnetic forces associated with the current. If, in addition, a static magnetic field is superimposed in the object, the induced eddy currents will be subjected to a force which can generate an acoustic wave in the object. Conversely, if the material in the object is set in motion due to a propagating acoustic wave, eddy currents will be induced in the metal as the wave travels through the magnetic field. These currents may be inductively detected by a similarly placed conductor associated with an electronic receiver. In ferromagnetic metals, an additional transduction mechanism due to magnetostriction can significantly increase the transducer efficiency.
EMATs can be configured to excite various forms of acoustic waves, such as surface waves, bulk shear waves, angle shear waves, and angle longitudinal waves. Some representative electromagnetic acoustic transducer structures, for example, are disclosed in U.S. Pat. Nos. 4,127,035, 4,104,922, and 3,850,028. These unique features have prompted the use of EMATs in ultrasonic inspection systems for detecting a number of different kinds of defects, including such flaws as cracks, surface abnormalities, weld defects, and corrosion-induced pitting and wall thinning. In one illustrative application, for example, EMATs have been employed in a system to measure the characteristics of an ultrasonic signal propagated through a defective region of a pipeline. Some characteristic properties of the EMAT, however, such as its inherently low level of signal generation, have led to undesirable consequences. These conventional systems, for example, tend to be excessively sensitive to local inhomogeneties in the pipe. Additional deficiencies with such systems include the presence of excessive levels of random electronic noise, impulsive noise produced by electronic machinery, and grain noise arising from the material being tested.
Consequently, a need has developed in the art for an improved technique for processing signals from an EMAT-equipped ultrasonic testing system.